Aim: To define the role of spiral CT in evolving an evidence-based 3D Conformal Radiotherapy (CRT) prostate protocol at Lincoln, UK.
Discussion: Tumour doses traditionally prescribed at this centre to the prostate planning target volume (PTV) (64 Gy in 32 fractions) cannot be further escalated without modification of existing technique and may currently be inadequate to obtain the highest probability of local control. Prostate CRT has been demonstrated to be well tolerated with both conventional and escalated doses, however as 3D CRT PTV margins are tightened, prostate position has to be reliably predicted to avoid geographic misses or unacceptable normal tissue toxicity. The question of prostate position variability might be addressed by sequential on-treatment spiral CT scans at this centre. Spiral CT offers specific advantages of speed, small detail conspicuity, and arbitrary axial reconstruction compared to conventional CT with no attached dose penalty. Spiral CT coupled to the next generation of radiotherapy treatment planning systems (RTPs) may soon replace the CT virtual-simulator. There are significant hardware discrepancies between some present generation CT couch tops and linac couch tops. Recently published CT studies that consider prostate position variability may be fundamentally and significantly flawed due to these couch top differences. Due to a paucity of reported evidence regarding immobilisation methods, a spiral CT study is warranted to assess efficacy of immobilization method for an evidence-based prostate protocol. Confirmative spiral CT research at this centre into prostate position variability is required to select adequate margins to form the PTV for an evidence-based 3D CRT prostate protocol. Such a spiral CT study could be integrated with the immobilisation study and may separate or define the correlation (which at present is both unclear and unreported) between pelvic immobilization and prostate position variability. Initial PTV margins defined by expanding the CTV in three dimensions using an ellipsoid with major axes 1.65 times one standard deviation of prostatic displacement reported in initial studies to obtain margins of 0.7 cm laterally, 0.7 cm cranio-caudally and 1.1 cm in the AP direction are presently indicated for this centre's evidence-based prostate protocol.
Conclusion: Spiral CT will provide the essential data set for 3D CRT planning for an evidence-based prostate protocol at Lincoln. Confirmative research using spiral CT is also warranted to assess daily prostate position variability and help define the prostate PTV for an evidence based prostate protocol at this centre.